1. Field of the Invention
The present invention relates to a surface-acoustic-wave (SAW) device using diamond.
2. Description of the Related Art
A SAW device is a device for selecting a specific frequency by impressing a radio frequency signal (RF signal) to an interdigital transducer (IDT) formed on a piezo electric body (a piezoelectric-material film, a piezoelectric-material board) to excite a surface acoustic wave, and converting a propagated signal into RF signal again. It has been widely used as a band pass filter or a resonator, for optical communications, satellite communications, mobile communications or the like, and is a key device for communication. Piezoelectric materials such as crystal, zinc oxide, lithium niobate (LN), lithium tantalate (LT) and the like have been used for a usual SAW device depending on the use.
It is known that an adaptive center frequency f0 of a SAW device is represented as f0=V/λ using a propagation velocity V of a surface acoustic wave and an interdigital transducer period λ. Then, in order to correspond to a demand for communication with high-frequency, the interdigital transducer period λ has been made shorter, i.e., an interdigital transducer pattern has been made finer (for example, see Japanese Patent Application Laid-open (KOKAI) No. 5-283970). However, there have been caused a problem that control of line width becomes difficult as a pattern gets finer and a problem that degradation progresses as a line width gets narrower.
Then, in order to meet a demand for a high-frequency which has rapidly arisen in a recent communication, it has becomes necessary not only to make an interdigital transducer pattern finer but to make a propagation velocity V of a surface acoustic wave higher. Specifically, a high-speed surface acoustic wave has been used with a substrate having a constitution wherein a film of a material with a high elasticity such as sapphire or diamond is formed on a base material, and a piezoelectric-material thin film is formed on the film (for example, see Japanese Patent Application Laid-open (KOKAI) No. 2-20910). Especially, a diamond has the highest elastic constant among the materials, and is only one material which can achieve a propagation velocity (V) of 10,000 m/s or more even when a piezoelectric material is laminated thereon.
However, when a diamond is used for a SAW device, there has been caused a problem that static electricity is generated on a surface of a substrate to break the substrate, in a process of manufacturing the SAW device, resulting in lowering of a device manufacture yield. Moreover, there has been caused a problem also at the time of real use of the produced SAW device, that a surface of the SAW device is charged with a prolonged use. The surface charge may naturally cause short-circuit, may cause a significant defects in process and performance, in relation to a spark discharge, and furthermore may cause even cracks in a SAW device.